Although various types of discharge lamps are known from the past, among high-pressure mercury lamps in which mercury is enclosed in a light emission tube thereof, especially a short arc type high pressure mercury lamp is used, for example, as a light source of light exposing apparatus for light exposure processing of a semiconductor wafer, a liquid crystal substrate, etc., since it has light emission property in which i rays with a wavelength of 365 nm or g rays with a wavelength of 435 nm are emitted. In such a short arc type high pressure mercury lamp, a high output is strongly demanded so that expected exposure processing can be performed at high processing efficiency.
In order to make a high pressure mercury lamp with a high output, rated power is usually raised, but the rated current also increases. As a result, there is a problem that the anode of the high-pressure mercury lamp which is turned on according to direct current lighting, readily becomes high in temperature so that the anode melts since the quantity of electrons which collide with the anode increases. Moreover, in a high-pressure mercury lamp, in which a pair of electrodes facing each other is provided above and below, the electrode located in the upside becomes high in temperature so as to sometimes melt, due to heat from an arc, in addition to influence of the heat convection within an arc tube thereof etc. Since when the tip portion of an electrode melts, not only an arc becomes unstable, but the substance which forms the electrode also evaporates, thereby adhering to the inner wall of an arc tube, so that there is a problem that the intensity of light emitted from the high-pressure mercury lamp falls.